Apparatus of this type is disclosed in prior GB specification No. 2030465 of the same inventor.
A further refinement disclosed in prior GB specification No. 2092905, corresponding to U.S. Pat. No. 4,377,480, also of the same inventor, is inclusion of compression means for additionally squeezing liquid from the filter cake formed on the upper run of the belt. The compression means takes the form of a drainage grid located below the upper run of the belt and pressed upwards by an inflatable bag so as to urge the belt and the filter cake thereon into contact with a pressure plate arranged above the belt. Prior GB specification No. 2090764, also of the same inventor, discloses supply of wash liquor, including wash liquor recycled from the vacuum means, to filter cake formed on the upper run of the belt as well as washing of the lower run of the filter belt by directing a jet of wash liquor onto a surface of the belt in the lower run.
Such apparatus has numerous applications in the separation of solids from liquids. It may be used, for example, for extraction of soluble solids from a mixture of solids and liquids. An example of this is the extraction of sugar from sugar beet or sugar cane, or the extraction of coffee, lignin, or dyes. Traditionally, the solids containing the soluble solids are broken up in the presence of a solvent and agitated or otherwise reacted for a given length of time before the insoluble solids are separated from the liquid phase, for example using filter apparatus of the type specified above.
It is normal to "wash" the insoluble solids with further supply of the same solvent or with another wash liquor in order to improve the extraction of soluble solids and/or the purity of the remaining insoluble solids, depending upon which is the commercially valuable product.
In the present economic climate, with greater awareness of need for economy and efficiency as well as minimising adverse effects on the environment, there is increasing demand for improved yield and higher purity of the commercially valuable product, whether soluble or insoluble solid. In apparatus of the type already described, the main requirement is to maximise extraction of soluble solids while using a minimum amount of solvent or wash liquor. Further, washing of the belt in its inoperative run must be as effective as possible, in order not to impair filtration efficiency and movement of the belt, but again the amount of solvent or wash liquor used for this should be minimised.
The conventional method of washing is to use a high pressure jet of wash liquor, as already mentioned. However, since the cleaning action of a liquid is proportional to its pressure the volume used has hitherto been determined by the level of pressure required rather than any other factor. Thus, large quantities of wash liquor have been used for washing filter cake and for subsequent belt washing. Subsequent extraction of relatively small quantities of either soluble or insoluble solids from such liquors has, in itself, been a problem.